One of the supplementary services of modern mobile communication systems is call forwarding which can be activated by a mobile subscriber. It is possible for the subscriber to determine various kinds of call forwarding functions. In an unconditional call forwarding, a mobile terminating call is always forwarded to a given call forwarding number. In a conditional call forwarding, a mobile terminating call is forwarded upon fulfilment of a specific condition; for example, when the subscriber is busy, when the mobile station cannot be reached, or when it is possible to establish a connection to the mobile station but the subscriber does not respond within a preset period of time.
In the most recent mobile communication systems, the subscriber data are stored in a subscriber register separate from the mobile services switching center. For example, in the pan-European digital mobile communication system GSM (Global System for Mobile Communications), the subscriber data are maintained in a network element separate from the mobile services switching center MSC, i.e. in a home location register HLR, and in a visitor location register VLR located at the switching center MSC but operationally separate from it. Information on the call forwarding is obtained, depending on the phase of the call, from either one of the registers as a result of a database enquiry by the mobile services switching center. Neither of the registers VLR or HLR, however, contain information on the phase of the forwarding call which is being established. Consequently, the register provides the mobile services switching center with a forwarding number even if the mobile services switching center already had another call forwarding call of the same subscriber in a set-up phase. The aforementioned leads to a situation where a mobile subscriber and another mobile subscriber or a subscriber of a public switched telephone network PSTN, (e.g. the same person from his work place telephone connected to the PSTN), place a call forwarding to each other, resulting in a third person who calls the same subscriber number or work place number creating an endless call forwarding, if the telephone network between the forwarding subscribers does not support transfer of the call forwarding information. The "endless call forwarding" finally results in all the trunk circuits between the switching centers being reserved for this one call until there is no free trunk circuit, whereby all the circuits reserved for the call are released. Both the unconditional and conditional call forwardings cause problems between a mobile subscriber and a PSTN subscriber, as well as between two mobile subscribers in different networks. Between mobile subscribers within the same network but in service areas of different switching centers, problems occur in connection with conditional call forwardings.